The hypothesis that free quarks can never be seen in isolation. It is a result of quantum chromodynamics, in which the property of asymptotic freedom means that the interactions between quarks get weaker as the distance between them gets smaller, and tends to zero as the distance between them tends to zero. Conversely, the attractive interactions between quarks get stronger as the distance between them gets greater, and the quark-confinement hypothesis is that the quarks cannot escape from one another. It is possible that at very high temperatures, such as those in the early universe, quarks may become free. The temperature at which this occurs is called the deconfinement temperature. The hypothesis of quark confinement has not been proved theoretically in a conclusive way although there is much evidence for it.